Optical amplifier for amplifying a wavelength division multiplexed (WDM) light including light in different wavelength bands

ABSTRACT

An optical amplifier, or optical repeater, for amplifying wavelength division multiplexed (WDM) light A first demultiplexer demultiplexes the WDM light into first and second lights corresponding to different wavelengths in the WDM light. First and second optical amplifiers amplify the first and second lights, respectively. A first multiplexer multiplexes the amplified first and second lights into a multiplexed light. A dispersion compensator compensates for dispersion in the multiplexed light. A second demultiplexer demultiplexes the dispersion compensated, multiplexed light into the first and second lights. Third and fourth optical amplifiers amplify the demultiplexed first and second lights, respectively. A second multiplexer multiplexes the amplified first and second lights from the third and fourth optical amplifiers into a WDM light. The optical amplifier can be configured so that the first and second lights travel through the dispersion compensator in opposite directions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on, and claims priority to, Japaneseapplication 10-249658, filed Sep. 3, 1998, in Japan, and which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical amplifier and opticalamplification method for amplifying wavelength division multiplexed(WDM) light which includes light in different wavelength bands.

2. Description of the Related Art

Research and development in the area of wavelength division multiplexed(WDM) optical communication systems has resulted in a steady increase inthe number of wavelengths being multiplexed together. In addition, thewavelength bands for transmission are being widened.

Furthermore, research and development is also advancing the developmentof WDM optical communication systems which utilize optical amplifiers aslinear repeaters. With such WDM optical communication systems, aplurality of signal lights in a wavelength band of, for example, 1.53 to1.56 μ(hereinafter referred to as a 1.55 μm band), can be collectivelyamplified with an optical amplifier, thereby enabling large-capacity andlong-distance light transmission with a simple construction.

In addition, optical communication systems which address band expansionof an optical amplifier have also been proposed. For example, opticalamplifiers which can amplify signal lights in a long wavelength band of,for example 1.57 to 1.60 μm (hereinafter referred to as a 1.58 μm band)have been proposed.

For example, FIG. 1 is a diagram showing a conventional opticalamplifier for amplifying WDM signal light which includes both signallight in a 1.55 μm band (1.53 to 1.56 μm) and signal light in a 1.58 μmband (1.57 to 1.60 μm). With typical optical amplifiers, in particularoptical fiber amplifiers, it is difficult to obtain an equal gain over awide band exceeding 60 nm. Therefore, the optical amplifier in FIG. 1divides the WDM signal light into, for example, two bands of 1.55 μm and1.58 μm, and obtains equal gains over the respective bands.

Referring now to FIG. 1, WDM signal light from a single optical fiber isdemultiplexed by a WDM coupler 1 into WDM signal lights of a 1.55 μmband and a 1.58 μm band. Then, the WDM signal lights of the 1.55 μm bandand the 1.58 μm band are directed to a 1.55 μm band optical fiberamplifier section 2 and a 1.58 μm band optical fiber amplifier section3, respectively. The respective WDM signal lights amplified by opticalfiber amplifier sections 2 and 3 are then multiplexed in a WDM coupler4, and output to a single optical fiber.

However, various problems can occur with an optical communication systemwhich transmits signal light over a wide wavelength band. For example,assume signal light of the 1.55 μm band is transmitted over a longdistance using an optical transmission path comprising, for example, asingle mode optical fiber (SMF) which transmits the wavelength close to1.3 μm with zero dispersion. In this case, there is a problem that thetransmitted waveform becomes distorted if the signal light istransmitted at a high transmission speed. This distortion is due to thewavelength dispersion characteristics of the optical transmission path.

For example, with a general 1.3 μm zero dispersion SMF, there is awavelength dispersion of approximately 18 ps/nm/km in the vicinity of1.55 μm. For example, in the case where a signal light of 1.55 μm istransmitted 50 km, then a wavelength dispersion of 900 ps/nm (=18ps/nm/km×50 km) accumulates. This is generally referred to as primarydispersion, and indicates that a delay difference of 900 ps perwavelength amplitude of 1 nm is produced.

Whether or not this delay difference exerts an influence on thetransmission characteristics is related to the time slot of the signallight. That is to say, in the case where the time slot of the signallight is sufficiently longer than the delay difference due to thewavelength dispersion, the influence on the transmission waveform isminimal. However when the time slot approaches the delay difference, theinfluence of the wavelength dispersion increases so that the waveformbecomes distorted. In general, it is considered that if the transmissionspeed of the signal light per unit wavelength exceeds approximately 2.5Gb/s, then compensation for wavelength dispersion is required. Forexample, in the case where the transmission speed of the signal light is10 Gb/s, the time slot becomes 100 ps, and the wavelength dispersion of900 ps/nm for the above mentioned case exerts a considerable influenceon the transmission characteristics.

To compensate for the wavelength dispersion characteristics of theoptical fiber transmission path, the light signal may be passed througha dispersion compensator having opposite wavelength dispersioncharacteristics to the transmission path. In the case of compensatingfor a wavelength dispersion of 900 ps/nm, a dispersion compensatorhaving a wavelength dispersion of −900 ps/nm is used. For example, adispersion compensating fiber (DCF) is widely used as such a dispersioncompensator.

However, in the case where compensation is performed with a wavelengthdispersion of 1.55 μm as a reference, as the wavelength band of thesignal light is increased, the compensation error increases as thedeviation of the wavelength from 1.55 μm increases.

For example, FIG. 2 is a diagram showing wavelength dispersioncharacteristics for a 1.3 μm zero dispersion SMF. As shown in FIG. 2,the wavelength dispersion characteristic of a 1.3 μm zero dispersion SMFhas an incline with respect to wavelength. As a result, for example, awavelength dispersion with respect to a signal light of 1.53 μm becomes18−Δ_(S) ps/nm/km, and a wavelength dispersion with respect to a signallight of 1.58 μm becomes 18+Δ_(L) ps/nm/km. Consequently in the casewhere 50 km transmission is performed, then even if a dispersioncompensator having a compensation amount of the abovementioned −900ps/nm is used, the Δ_(S)×50 ps/nm component is excessively compensatedfor with respect to the signal light of 1.53 μm, while the Δ_(L)×50ps/nm component is insufficiently compensated for with respect to thesignal light of 1.58 μm. The wavelength dispersion produced due to thissituation where the wavelength dispersion characteristics of the opticalfiber transmission path have an incline with respect to the wavelengthis generally referred to as secondary dispersion, and when the number ofwavelengths of the signal light is large and the wavelength band iswide, it is necessary to perform compensation not only for primarydispersion but also for secondary dispersion.

As mentioned above, a high speed WDM optical communication system with atransmission speed per unit wavelength exceeding 2.5 Gb/s, using a 1.55μm band or a 1.58 μm band as the band for wavelength divisionmultiplexed signal light is, currently being developed. In realizingsuch a system, an important consideration is how to compensate for theprimary and secondary wavelength dispersion to improve efficiency.Furthermore, it is considered that when wavelength dispersioncompensation in the above mentioned wide wavelength band is collectivelyperformed, a signal light of large power is transmitted to thedispersion compensator. Therefore, for example, a nonlinear opticaleffect such as cross-phase modulation (XPM) or four-wave mixing (FWM) islikely to occur, so that there is the likelihood of degradation of thetransmission characteristics.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anoptical amplifier and optical communication system of simpleconstruction which can compensate for wavelength dispersion with respectto WDM signal light of a wide band. Furthermore, it is an object of thepresent invention to provide a wavelength dispersion compensation methodwhich reduces the probability of the occurrence of nonlinear opticaleffects when transmitting WDM signal light.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

Objects of the present invention are achieved by providing an apparatuswhich demultiplexes light into a plurality of wavelength bands accordingto optical wavelength and respectively amplifies the demultiplexedlights of the respective wavelength bands with a plurality of opticalamplifiers. More specifically, the apparatus includes a plurality ofoptical amplifiers for amplifying the plurality of wavelength bands, awavelength dispersion compensator for compensating for wavelengthdispersion, and an optical multiplexor/demultiplexor. The opticalmultiplexor/demultiplexor takes out and multiplexes lights of respectivewavelength bands from inside the respective optical amplifiers, and theninputs the multiplexed light into the wavelength dispersion compensator,and also demultiplexes the light output from the wavelength dispersioncompensator into the respective wavelength bands and returns thedemultiplexed lights to the respective optical amplifiers.

Preferably the respective optical amplifiers incorporate a pre-stageoptical amplifier section and a post-stage optical amplifier sectionconnected together in series, and the optical multiplexor/demultiplexortakes out light from between the pre-stage optical amplifier section andthe post-stage optical amplifier section of each of the respectiveoptical amplifiers. By having such a construction, light signals of anappropriate power are input to the wavelength dispersion compensator.Therefore, the occurrence of nonlinear optical effects and degradationof the optical SN ratio is suppressed.

Moreover, as a specific construction for the respective opticalamplifiers, a variable optical attenuator may be provided between thepre-stage optical amplifier section and the opticalmultiplexor/demultiplexor. Furthermore, with the variable opticalattenuator, preferably the light attenuation amount is controlled sothat the power of the light output from the post-stage optical amplifiersection becomes a fixed level. Preferably, the gains of the pre-stageoptical amplifier section and the post-stage optical amplifier sectionare controlled to be constant.

In addition, preferably, the wavelength dispersion compensator is adispersion compensating fiber, and the optical multiplexor/demultiplexorhas two optical multiplexing/demultiplexing devices respectivelyconnected to both end portions of the dispersion compensating fiber.Lights of adjacent wavelength bands of the lights first taken out fromthe respective optical amplifiers are respectively input to thedispersion compensating fiber via the different opticalmultiplexing/demultiplexing devices. Moreover, the respective opticalmultiplexing/demultiplexing devices may be optical circulators.

With such a construction, the signal lights of adjacent wavelength bandsare respectively input from the respective end portions of thedispersion compensating fiber via different opticalmultiplexing/demultiplexing devices, and propagated in mutuallydifferent directions inside the dispersion compensating fiber. As aresult, the situation where signal light of large power is concentratedat a specific portion of the dispersion compensating fiber is avoided,and the propagation directions of the lights of adjacent wavelengthbands are opposite. Therefore, the probability of the occurrence ofnonlinear optical effects in the dispersion compensating fiber isfurther reduced.

Furthermore, objects of the present invention are achieved by providingan apparatus which demultiplexes light into a plurality of wavelengthbands according to optical wavelength and respectively amplifies thedemultiplexed lights of the respective wavelength bands with a pluralityof optical amplifiers, and then multiplexes the amplified lights. Theapparatus further comprises a first wavelength dispersion compensatorfor compensating for wavelength dispersion with a dispersion amount fora previously set wavelength as a reference, an opticalmultiplexor/demultiplexor which first takes out and multiplexes lightsof respective wavelength bands sent to the respective optical amplifiersand then inputs the multiplexed light into the first wavelengthdispersion compensator, and also demultiplexes the light output from thefirst wavelength dispersion compensator into the respective wavelengthbands and returns the demultiplexed lights to the respective opticalamplifiers. A second wavelength dispersion compensator separatelycompensates for the wavelength dispersion which is not completelycompensated for by the first wavelength dispersion compensator, for eachrespective wavelength band.

With such a construction, in the case where wavelength dispersion forthe respective wavelength bands cannot be compensated for by a singlewavelength dispersion compensator, a first wavelength dispersioncompensator for compensating for wavelength dispersion with a dispersionamount for a previously set wavelength as a reference is provided, and asecond wavelength dispersion compensator for separately compensating forthe wavelength dispersion which is not completely compensated for by thefirst wavelength dispersion compensator for each respective wavelengthband is provided. In this way, an optical amplifier which can performwavelength dispersion compensation with respect to the respectivewavelength bands is realized with a comparatively simple construction.

Objects of the present invention are also achieved by providing anoptical communication system for multiply repeating and transmittingwavelength division multiplexed signal light using a plurality ofoptical amplifier repeaters connected in series via an opticaltransmission path. The plurality of optical amplifier repeaters areoptical amplifiers which divide the light into a plurality of wavelengthbands according to optical wavelength, and respectively amplify thedemultiplexed lights of the respective wavelength bands with a pluralityof optical amplifiers and then multiplex the amplified lights. Theplurality of optical amplifier repeaters have first and secondconstructions.

The optical amplifier repeater of the first construction includes aplurality of optical amplifiers for amplifying a plurality of wavelengthbands, respectively. A wavelength dispersion compensator compensates forwavelength dispersion of the wavelength division multiplexed signallight with a dispersion amount for a previously set wavelength as areference. An optical multiplexor/demultiplexor takes out andmultiplexes lights of respective wavelength bands sent to the respectiveoptical amplifiers and then inputs the multiplexed light into thewavelength dispersion compensator, and also demultiplexes the lightoutput from the wavelength dispersion compensator into the respectivewavelength bands and returns the demultiplexed lights to the respectiveoptical amplifiers.

The optical amplifier repeater of the second construction includes aplurality of optical amplifiers for amplifying a plurality of wavelengthbands, respectively. A first wavelength dispersion compensatorcompensates for wavelength dispersion of the wavelength divisionmultiplexed signal light with a dispersion amount for a previously setwavelength as a reference. A second wavelength dispersion compensatorseparately compensates for wavelength dispersion which is not completelycompensated for by the first wavelength dispersion compensator, for eachrespective wavelength band. Preferably, the first construction opticalamplifier repeater and the second construction optical amplifierrepeater are positioned alternately one after the other.

With such a construction, when the wavelength division multiplexedsignal light, which is multiply repeated and transmitted by theplurality of optical amplifier repeaters, passes through the opticalamplifier repeater of the first construction, the wavelength dispersioncharacteristics of the optical transmission path are compensated for byone wavelength dispersion compensator. With this wavelength dispersioncompensation, the dispersion amount for the previously set wavelength ismade a reference, and sufficient dispersion compensation is notperformed for all of the respective wave bands. Therefore, when thewavelength division multiplexed signal light passes through the opticalamplifier repeater of the second construction, the wavelength dispersionwhich has not been completely compensated for is separately compensatedfor each respective wavelength band. As a result, the wavelengthdispersion characteristics of the optical transmission path can becompensated for all of the plurality of optical amplifier repeaters.Consequently, since the optical amplifier repeater of the firstconstruction has a simple construction, it is easy to realize an opticalcommunication system incorporating a wavelength dispersion function.

Furthermore, objects of the present invention are achieved by providinga wavelength dispersion compensation method in which wavelength divisionmultiplexed signal light is demultiplexed into a plurality of wavelengthbands according to wavelength. The lights of adjacent wavelength bandsof the demultiplexed lights are respectively input from different endportions of the dispersion compensating fiber, and the lights ofrespective wavelength bands respectively output from the respective endportions of the dispersion compensating fiber are multiplexed.

With such a construction, the lights of adjacent wavelength bands arerespectively input from the different end portions of the dispersioncompensating fiber and propagated in mutually different directionsinside the dispersion compensating fiber. As a result, the situationwhere signal light of large power is concentrated at a specific portionof the dispersion compensating fiber is avoided, and the propagationdirections of the lights of adjacent wavelength bands are opposite.Therefore the probability of the occurrence of the nonlinear opticaleffect in the dispersion compensating fiber is further reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe preferred embodiments, taken in conjunction with the accompanyingdrawings of which:

FIG. 1 (prior art) is a diagram showing a conventional optical amplifierfor amplifying WDM signal light of a 1.55 μm band and a 1.58 μm band.

FIG. 2 (prior art) is a diagram showing wavelength dispersioncharacteristics for a 1.3 μm zero dispersion SMF.

FIG. 3 is a diagram showing an optical amplifier, according to anembodiment of the present invention.

FIG. 4 is a diagram showing an optical amplifier, according to anadditional embodiment of the present invention.

FIGS. 5(A) and 5(B) are diagrams showing changes in optical power insidea dispersion compensating fiber (DCF) of signal light in the 1.58 μmband and the 1.55 μm band, respectively, in the optical amplifier ofFIG. 4, according to an embodiment of the present invention.

FIG. 6 is a diagram showing an optical amplifier, according to a furtherembodiment of the present invention.

FIG. 7 is a diagram showing an optical amplifier, according to a stillfurther embodiment of the present invention.

FIG. 8 is a diagram showing an optical amplifier, according to anembodiment of the present invention.

FIG. 9 is a diagram showing an optical amplifier, according to anadditional embodiment of the present invention.

FIG. 10 is a diagram showing an optical communication system, accordingto an embodiment of the present invention.

FIG. 11 is a diagram showing an even number optical amplifier repeater,according to an embodiment of the present invention, as used in theoptical communication system of FIG. 10.

FIG. 12 is a diagram showing an even number optical amplifier repeater,according to an additional embodiment of the present invention, as usedin the optical communication system of FIG. 10.

FIG. 13 is a diagram showing an optical fiber amplifier with and an AGCcircuit, according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tolike elements throughout.

FIG. 3 is a diagram showing an optical amplifier, according to anembodiment of the present invention. Referring now to FIG. 3, a WDMsignal light from, typically, a SMF (not illustrated) includes signallight in a 1.55 μm band and a 1.58 μm band. A WDM coupler 1demultiplexes the WDM signal light according to the wavelength bands,and the signal light in the 1.55 μm band is sent to a 1.55 μm bandoptical fiber amplifier section 2, and the signal light in the 1.58 μmband is sent to a 1.58 μm band optical fiber amplifier section 3. As anexample, the signal light in the 1.55 μm band might include signallights of thirty two waves multiplexed in a wavelength band from 1535 nmto 1565 nm. As an example, the signal light in the 1.58 μband mightinclude thirty two waves multiplexed in a wavelength band from 1575 nmto 1605 nm. However, the WDM signal light of the present invention isnot limited to these example wavelength arrangements, these wavelengths,or these wavelength bands, and many variations are possible.

The signal light input to 1.55 μm band optical fiber amplifier section 2is amplified to a predetermined level in an optical fiber amplifier 20.With optical fiber amplifier 20, the gain is controlled to be constantby an automatic gain control (AGC) circuit 21. Therefore, the wavelengthcharacteristics of the gain are constant for a wide input range. Then,the signal light output from optical fiber amplifier 20 is sent to avariable optical attenuator (ATT) 22 and attenuated in accordance withan optical attenuation value controlled by an automatic level control(ALC) circuit 23. The signal light which has passed through variableoptical attenuator 22 is then sent to a WDM coupler 10.

The signal light input to 1.58 μband optical fiber amplifier section 3is amplified to a predetermined level in an optical fiber amplifier 30.With optical fiber amplifier 30, the gain is controlled to be constantby an automatic gain control (AGC) circuit 31. Therefore, the wavelengthcharacteristics of the gain are constant for a wide input range. Then,the signal light output from optical fiber amplifier 30 is sent to avariable optical attenuator (ATT) 32 and attenuated in accordance withan optical attenuation value controlled by an automatic level control(ALC) circuit 33. The signal light which has passed through variableoptical attenuator 32 is then sent to WDM coupler 10.

In WDM coupler 10, the signal lights of the respective wavelength bandssent from respective variable optical attenuators 22 and 32 aremultiplexed and then sent to a DCF 11. By passing the multiplexed signallight through DCF 11, wavelength dispersion in the 1.55 μm band and the1.58 μm band is compensated collectively. The signal light receives aloss from DCF 11 so that the power at the output of DCF 11 is reduced byapproximately 10 dB compared to the power at the input of DCF 11.

Therefore, the signal light output from DCF 11 is again demultiplexedinto the 1.55 μm band and the 1.58 μm band by a WDM coupler 12. Thesignal light in the 1.55 μm band is sent to post-stage optical fiberamplifier 24. Similarly, the signal light in the 1.58 μband is sent topost-stage optical fiber amplifier 34.

With optical fiber amplifier 24, the gain is controlled to be constantby an automatic gain control (AGC) circuit 25. Similarly, with opticalfiber amplifier 34, the gain is controlled to be constant by anautomatic gain control (AGC) circuit 35. Therefore, with the respectiveoptical fiber amplifiers 24 and 34, the signal lights of thecorresponding wavelength bands are amplified at a constant gain to apredetermined level.

A part of the amplified signal from optical fiber amplifier 24 isreturned to ALC circuit 23 to control variable optical attenuator 22 tomaintain the output light level of optical fiber amplifier section 2constant. Similarly, a part of the amplified signal from optical fiberamplifier 34 is returned to ALC circuit 33 to control variable opticalattenuator 32 to maintain the output light level of optical fiberamplifier section 3 constant.

Then, the signal lights output from optical fiber amplifier sections 2and 3, being controlled to a constant level, are sent to a WDM coupler4. With WDM coupler 4, the signal lights of the 1.55 μm band and the1.58 μm band for which amplification and wavelength dispersioncompensation has been effected, are multiplexed and sent to an opticalcoupler 13. In optical coupler 13, the WDM signal light from WDM coupler4 is output as the output light for the optical amplifier, and a partthereof is branched for monitor light.

With this monitor light, for example, the spectrum of the WDM signallight output from the optical amplifier can be monitored and used foradjustment and the like of the operating conditions of optical fiberamplifier sections 2 and 3 so that the signal light power of therespective wavelength bands becomes approximately equal. The monitorlight is particularly useful to enable verification of the connectionconditions, or adjustment of the operating conditions, for example, inthe case where the optical amplifier is to be furnished with expansioncapability. For example, via the monitor light, the amplificationprovided by 1.58 μm band optical fiber amplifier section 3 can beincreased with an increase in the number of multiplexed wavelengths.Alternately, if the respective wavelength light from the monitor lightis extracted, and the respective signal waveforms monitored, this canalso be used to verify whether or not wavelength dispersion compensationis being effectively performed.

Preferably, DCF 11 is a secondary dispersion compensation type which isable to collectively compensate for the wavelength dispersioncharacteristics with respect to a 1.55 μband and a 1.58 μm band of a 1.3μm zero dispersion SMF.

In FIG. 3, 1.55 μm band optical fiber amplifier section 2 and 1.58 μmband optical fiber amplifier section 3 are each two-stage amplifiers.Before being sent to the post-stage optical amplifier sections, therespective wavelength band signal lights are taken out, multiplexed inWDM coupler 10, and then sent to DCF 11. The signal lights which havepassed through DCF 11 are again demultiplexed into a 1.55 μm band and a1.58 μm band in WDM coupler 12, and then returned to the post-stageoptical amplifier sections of the respective optical fiber amplifiersections 2 and 3.

In 1.55 μm band optical fiber amplifier section 2, optical fiberamplifier 20 serves as a pre-stage optical amplifier section and opticalfiber amplifier 24 serves as a post-stage optical amplifier section. Asindicated above, respective optical fiber amplifiers 20 and 24 areprovided with AGC circuits 21 and 25, respectively, for controlling theamplification gain to be constant. Moreover, as indicated above,variable optical attenuator 22 is provided ALC circuit 23 forcontrolling the variable attenuation amount so that the power level ofthe signal light output from post-stage optical fiber amplifier 24 isconstant. Similarly, as indicated above, variable optical attenuator 32is provided ALC circuit 33 for controlling the variable attenuationamount so that the power level of the signal light output frompost-stage optical fiber amplifier 34 is constant.

Optical amplifiers 20 and 24 are optical amplifiers where, for example,a signal light of a 1.55 μm band is sent to an erbium doped fiber (EDF)(not illustrated) which is in an excited state due to excitation lightof a 0.98 μm band or a 1.48 μm band, to thereby amplify respectivewavelength light of the 1.55 μm band at approximately equal gains.However, optical amplifiers 20 and 24 are not intended to be limited tothis construction, or to these specific excitation light wavelengths, orto using an EDF. Instead, many different types of optical amplifiers canbe used.

As indicated above, 1.58 μm band optical fiber amplifier section 3comprises optical fiber amplifiers 30 and 34 provided with AGC circuits31 and 35, respectively, and variable optical attenuator (ATT) 32provided with ALC circuit 33. Known 1.58 μm band optical fiberamplifiers can be used, for example, as optical fiber amplifiers 30 and34. In such optical amplifiers, an EDF can be employed as anamplification medium. In order to produce an optical amplificationeffect in the 1.58 μm band, for example, the EDF length is made longerthan that for an 1.55 μm band optical fiber amplifier.

To explain briefly the operating theory for an 1.58 μm band opticalfiber amplifier, the erbium atom inside the EDF is excited by excitationlight of a 1.48 μm band or a 0.98 μm band, and a 1.55 μm bandspontaneous emission light (ASE) is produced in the former half portionof the EDF. This 1.55 μm band ASE is reabsorbed in the latter halfportion of the EDF to thereby produce induced emission in the 1.58 μmband. Since the cross-sectional area of the induced emission in the 1.58μm band is smaller than that in the 1.55 μm band, and it is necessary toproduce a sufficiently large 1.55 μm band ASE. Therefore, for example,the fiber length of the EDF is increased to thereby realize opticalamplification in the 1.58 μm band. However, optical fiber amplifiers 30and 34 are not intended to be limited to this construction. Instead,many different types of optical amplifiers can be used.

WDM couplers 10 and 12 are, for example, couplers where, as with WDMcouplers 1 and 4, when lights in the 1.55 μm band and the 1.58 μm bandare input to prescribed ports, these lights are multiplexed and outputfrom a single port. On the other hand, when the multiplexed light of the1.55 μm band and the 1.58 μm band is input, this light is demultiplexedinto lights of the 1.55 μm band and the 1.58 μm band and output from theprescribed ports. Such couplers are known, and the present invention isnot limited to any specific coupler.

In the optical amplifier of FIG. 3, DCF 11 is preferably a dispersioncompensating fiber of the secondary dispersion compensation type with awide band including, for example, the 1.55 μm band and the 1.58 μm band.More specifically, a dispersion compensating fiber which has awavelength dispersion characteristic with a negative sign and a negativeincline in contrast with the wavelength dispersion characteristics ofthe 1.3 μm zero dispersion SMF shown in FIG. 2 is used. With DCF 11, thelosses are greater than with the SMF. Here, for example, with a lengthof approximately 10 km of DCF, there is a loss of approximately 10 dB.However, the present invention is not limited to any specific type ofDCF, or any specific wavelength bands.

In FIG. 3, optical coupler 13 is provided after WDM coupler 4 on theoutput side. Optical coupler 13 is for taking out, at a branch ratio of,for example, 10:1, a portion of the WDM signal light which has beenmultiplexed in WDM coupler 4 to become the output light of the opticalamplifier, in order to obtain monitor light. Of course, optical coupler13 is not limited to this branching ratio.

As illustrated in FIG. 3, the input and output of 1.55 μm band opticalamplifier 2 preferably serve as a connector interface. If this is done,then by simply interchanging the input and output of 1.55 μm bandoptical amplifier 2, a bi-directional optical amplifier can be realized.(See, for example, the bi-directional optical amplifier in FIG. 4,discussed later.) This is possible because, with this construction, theALC is realized by a variable optical attenuator for both the 1.55 μmband and the 1.58 μm band. That is, even if the span losses on the inputside and the output side of a point setting the optical amplifier aredifferent, since the variable optical attenuator absorbs thefluctuations in the span losses, selection in a single direction or twodirections becomes possible.

With the optical amplifier in FIG. 3, WDM signal light can betransmitted at high speed. This WDM signal light is amplified bydemultiplexing into two wavelength bands, then by incorporating a DCFwhich can compensate for wavelength dispersion for the respectivewavelength bands collectively, inside the optical amplifier. Therefore,an optical amplifier provided with a wavelength dispersion compensationfunction can be realized with a simple construction. With this, comparedto an example where respective DCFs are provided for each of the 1.55 μmband and the 1.58 μm band, the number of high cost DCFs can be reduced.Hence, it is possible to reduce the price of an optical amplifierprovided with a wavelength dispersion compensation function.

With the optical amplifier in FIG. 3, the construction is for a casewhere the respective signal lights prior to input to the respectiveoptical fiber amplifiers 24 and 34 for the respective wavelengths aretaken out and sent to DCF 11. However the present invention is notlimited to this. For example, a construction is also possible where thesignal light at optional positions of the respective optical fiberamplifiers 2 and 3, for example, the signal light output from therespective optical fiber amplifiers 24 and 34, is taken out and sent toDCF 11. However, if consideration is given to the influence of nonlinearoptical effects, and to noise characteristics etc., ideally the signallight should be taken out from between the pre-stage optical amplifiersection and the post-stage optical amplifier section.

That is, in the case where the respective signal lights amplified by thepost-stage optical amplifier section are taken out, then signal light ofan extremely large power is input to a DCF with a small core diameter.Hence, the probability of the occurrence of nonlinear optical effects ishigh. On the other hand, in the case where respective signal lightsprior to being amplified by the pre-stage optical amplifier section aretaken out, signal light of a comparatively low power is input to thehigh loss DCF. Therefore, the optical SN ratio is degraded.Consequently, sending signal light to the DCF, after amplifying by thepre-stage optical amplifier section and prior to being amplified by thepost-stage optical amplifier section, is preferable.

FIG. 4 is a diagram showing an optical amplifier, according to anembodiment of the present invention. With the optical amplifier in FIG.3, the respective signal lights of the 1.55 μm band and the 1.58 μm bandwere transmitted in the same direction. By contrast, with the opticalamplifier in FIG. 4, the signal lights of the respective wavelengthbands are transmitted in mutually opposite directions. Morespecifically, the transmission direction for the 1.55 μm band signallight is in the opposite direction (from the right to left) in FIG. 4,as compared to that in FIG. 3. Moreover, the input/output positions of1.55 μm band optical fiber amplifier section 2 are reversed in FIG. 4,as compared to that in FIG. 3. The remaining components in FIG. 4 arethe same as that in FIG. 3.

Referring now to FIG. 4, the signal light of the 1.55 μm band is sent to1.55 μm band optical fiber amplifier section 2 via WDM coupler 4.Furthermore, the signal light of the 1.58 μm band is sent to 1.58 μmband optical fiber amplifier section 3 via WDM coupler 1. The signallights of the respective wavelength bands are respectively amplified byoptical fiber amplifiers 20 and 30, and respectively attenuated byvariable optical attenuators 22 and 32. The 1.55 μm band signal lightoutput from variable optical attenuator 22 is input from the port on theright side (in FIG. 4) of DCF 11 via WDM coupler 12. On the other hand,the 1.58 μm band signal light output from variable optical attenuator 32is input from the port on the left side (in FIG. 4) of DCF 11 via WDMcoupler 10. The signal lights of the respective wavelength bands inputto DCF 11 are propagated in mutually opposite directions inside DCF 11to thereby be subjected to wavelength dispersion compensation.

The signal lights of the respective wavelength bands which have passedthrough DCF 11 are sent to the respective optical fiber amplifiers 24and 34 via WDM couplers 10 and 12, and amplified to a predeterminedlevel. Then, the 1.55 μband signal light output from optical fiberamplifier 24 is output to an external SMF via WDM coupler 1, while the1.58 μm band signal light output from optical fiber amplifier 34 isoutput to the external SMF via WDM coupler 4.

The optical amplifier in FIG. 4 differs from the optical amplifier inFIG. 3 in that the probability of occurrence of nonlinear opticaleffects inside DCF 11 is kept low. For example, since the loss in DCF 11is large at around 10 dB, the power of the respective signal lightspropagated inside DCF 11 changes.

For example, FIGS. 5(A) and 5(B) are diagrams showing changes in opticalpower inside DCF 11 of signal light in the 1.58 μm band and the 1.55 μmband, respectively, in the optical amplifier of FIG. 4. As a consequenceof the changes in optical power inside DCF 11 as illustrated in FIGS.5(A) and 5(B), even though the powers of the respective input signallights increase, the situation where a large optical power isconcentrated at a specific part inside DCF 11 does not arise.Furthermore, the propagation directions for the respective signal lightsinside DCF 11 also become opposite directions. Due to the multipliereffect of this, it is difficult for the nonlinear optical effects tooccur.

In the above manner, with the optical amplifier in FIG. 4, even in thecase where signal lights of two wavelength bands are transmitted in twodirections, an optical amplifier with a wavelength dispersioncompensation function can be realized with a simple construction. Inaddition, since the signal lights of the respective wavelength bands areinput from different ports of the DCF, then the probability ofoccurrence of nonlinear optical effects in the DCF can be reduced, sothat more stable transmission characteristics can be obtained.

In FIG. 4, the signal lights of respective wavelength bands are input toDCF 11 via WDM couplers 10 and 12. However the present invention is notlimited to this.

For example, FIG. 6 is a diagram illustrating an optical amplifier,according to an additional embodiment of the present invention. In FIG.6, optical circulators 14 and 15 are used instead of WDM couplers 10 and12.

Optical circulators 14 and 15 are, for example, known optical circuitelements having, for example, three terminals t₁, t₂ and t₃, whereinlight which proceeds between the respective terminals in the sequentialdirection t1→t2, t2→t3, t3→t1, has a low loss while light which proceedsin the opposite direction has a high loss.

FIG. 7 is a diagram showing an optical amplifier, according to anadditional embodiment of the present invention. The optical amplifier inFIG. 7 is an improved example of the optical amplifier in FIG. 3, inthat, for example, in the case where the transmission directions for twowavelength bands are in the same direction, the occurrence of nonlinearoptical effects in the DCF 11 is reduced.

Referring now to FIG. 7, the respective connections are changed in theoptical amplifier of FIG. 3, so that the signal light output fromvariable optical attenuator 22 of 1.55 μm band optical fiber amplifiersection 2 is input to DCF 11 via WDM coupler 12, and the 1.55 μm bandsignal light which is passed through DCF 11 is sent to optical fiberamplifier 24 via WDM coupler 10.

With the optical amplifier of FIG. 7, the signal lights output fromvariable optical attenuators 22 and 32 of the respective wavelengthbands are respectively input to different ports of DCF 11 and propagatedinside DCF 11 in mutually opposite directions. As a result, even thoughthe powers of the lights respectively input to DCF 11 increase, thesituation where a large optical power is concentrated in the vicinity ofone port does not arise, and the propagation directions of therespective signal lights are opposite. Hence it is difficult for thenonlinear optical effects to occur.

In the above manner, with the optical amplifier of FIG. 7, even thoughthe transmission directions for the respective wavelength bands are inthe same direction, by merely making the propagation directions insideDCF 11 opposite, the probability of the occurrence of the nonlinearoptical effects can be reduced. Hence, it is possible to obtain morestable transmission characteristics.

With the optical amplifier in FIG. 7, the signal lights of therespective wavelength bands are input to DCF 11 via WDM couplers 10 and12. However, as described above, instead of WDM couplers 10 and 12,optical circulators may be used.

For example, FIG. 8 is a diagram illustrating an optical amplifieraccording to an embodiment of the present invention. The opticalamplifier in FIG. 8 is similar to that in FIG. 7, but uses circulators14 and 15 instead of WDM couplers 10 and 12.

With the optical amplifiers in FIGS. 3-8, a single DCF is able tocollectively compensate for the wavelength dispersion for the respectivewavelength bands. However, in some cases, a single DCF may not be ableto appropriately compensate for the primary and secondary wavelengthdispersion in a wide band. For example, different wavelengths typicallyrequire different amounts of dispersion compensation, so that a singleDCF may not be able to appropriately compensate for dispersion over awide band. For example, as indicated by FIG. 2, more dispersioncompensation is needed with the 1.58 μm band as compared to thatrequired for the 1.55 μm band.

To address this situation, FIG. 9 is a diagram showing an opticalamplifier, according to an additional embodiment of the presentinvention. More specifically, FIG. 9 shows a technique for providing anextra DCF for a respective band requiring additional dispersioncompensation, such as the 1.58 μm band.

With the optical amplifier in FIG. 9, instead of DCF 11 (such as that inFIG. 3), a DCF 11′ is used. Moreover, assuming that DCF 11′ does notprovide sufficient dispersion compensation for the 1.58 μm band, anadditional DCF 11″ is provided between variable optical attenuator 32and WDM coupler 10. Construction other than this is the same that inFIG. 3.

With DCF 11′, the compensation amount is set, for example, with thedispersion amount for a previously set wavelength in the 1.55 μm band asa reference, and primary and secondary wavelength dispersioncompensation for the 1.55 μm band is possible. However, wavelengthdispersion compensation for the 1.58 μm band cannot be sufficientlyperformed so that a not yet compensated portion is produced. With DCF11″, the compensation amount is adjusted so as to correspond to the notyet compensated portion of DCF 11′. Consequently, by passing the signallight of the 1.58 μm band through DCF 11″ and DCF 11′, the primary andsecondary dispersion is compensated for.

In this way, in the case where dispersion compensation for all of thewavelength bands cannot be collectively performed with one DCF, byproviding the two DCFs, DCF 11″ and DCF 11′, dispersion compensation forall wavelength bands can be performed with a single optical amplifier ofa comparatively simple construction.

With the optical amplifier of FIG. 9, DCF 11″ is provided betweenvariable optical attenuator 32 and WDM coupler 10. However the locationof DCF 11″ is not limited to this, and may be provided, for example,between WDM coupler 12 and optical fiber amplifier 34.

With the optical amplifier in FIG. 9, wavelength dispersion compensationis possible with respect to all bands, inside one optical amplifier.

While FIG. 9 shows an optical amplifier for amplifying two wavelengthbands, the present invention is clearly applicable to optical amplifiersfor amplifying more than two wavelength bands. In this case, forexample, different DCFs, such as DCF 11″, can be provide for eachwavelength band requiring additional dispersion compensation.

FIG. 10 is a diagram showing an optical communication system, accordingto an embodiment of the present invention. With the opticalcommunication system of FIG. 10, wavelength dispersion compensation forall wavelength bands is made possible by a plurality optical amplifierrepeaters connected in series via SMFs.

Referring now to FIG. 10, signal lights transmitted from opticaltransmitters T×1 to T×N for generating signal light of N waves withdifferent wavelengths located, for example, in the 1.55 μm band and the1.58 μm band, are multiplexed in a multiplexer (MUX) 50 and output to asingle SMF 52. After being transmitted while being successivelyamplified by n optical amplifier repeaters AMP1 to AMPn, the wavelengthdivision multiplexed signal light is demultiplexed for each respectivewavelength by a demultiplexer (DMUX) 54 and received by correspondinglight receivers R×1 to R×N.

Of optical amplifier repeaters AMP1 to AMPn, for example, the odd numberoptical amplifiers AMP1, AMP3 . . . , are constructed as illustrated,for example, in FIG. 3, or in various of the other embodiments of thepresent invention discussed above. However, it is assumed that therespective DCFs 11 used in these optical amplifier repeaters AMP1, AMP3,. . . , do not have a sufficiently wide band which can collectivelycompensate for the wavelength dispersion for the wavelength bands, andhence insufficient compensation or excessive compensation occurs.

On the other hand, the even number optical amplifier repeaters AMP2,AMP4, . . . serving as compensation optical amplifier repeaters, have adifferent construction.

More specifically, FIG. 11 is a diagram showing an even number opticalamplifier repeater, according to an embodiment of the present invention,as used in the optical communication system of FIG. 10. Referring now toFIG. 11, with the even number optical amplifier repeaters AMP2, AMP4, .. . , the construction is such that, with the 1.55 μm band, the signallights output from variable optical attenuator 22 travel through DCF 16and then to optical fiber amplifier 24. Similarly, with the 1.58 μmband, the signal lights output from variable optical attenuator 32travel through DCF 17 and then to optical fiber amplifier 34. Thus, DCFs16 and 17 are provided for separately compensating for the wavelengthdispersion in the upstream SMF and the wavelength dispersion whichcannot be compensated for by the upstream optical amplifier repeaters,for each respective band.

FIG. 12 is a diagram showing an even number optical amplifier repeater,according to an additional embodiment of the present invention, as usedin the optical communication system of FIG. 10. The optical amplifier inFIG. 12 is different than that in FIG. 11 in that optical coupler 13 isomitted.

To give a specific example for the respective DCFs, with the DCF used inthe odd number optical amplifier repeaters AMP1, AMP3, . . . , if thisis one where the dispersion amount for the wavelength of approximately1.57 μm being approximately the center for the used wavelength band ismade a reference, then excessive compensation is produced for the 1.55μm band and insufficient compensation is produced for the 1.58 μm band.In this case, for the 1.55 μm band DCF 16 used in the even numberoptical amplifier repeaters AMP2, AMP4, . . . , one having a positivewavelength dispersion corresponding to the excessive compensationportion is used, and for the 1.58 μm band DCF 17, one having a negativewavelength dispersion corresponding to the insufficient compensationportion is used.

Alternatively, with the DCF used in the odd number optical amplifierrepeaters AMP1, AMP3, . . . , if this is one where the dispersion amountfor the wavelength of approximately the 1.53 μis made a reference, theninsufficient compensation is produced respectively for the 1.55 μm bandand the 1.58 μm band. In this case, for the respective DCFs 16 and 17used in the even number optical amplifier repeaters AMP2, AMP4, . . . ,ones having a negative wavelength dispersion respectively correspondingto the insufficient compensation portions for the respective wavelengthbands are used.

With such an optical communication system, for the odd number opticalamplifier repeaters AMP1, AMP3, . . . , it is possible to have acomparatively simple construction using a single DCF. The wavelengthdispersion which cannot be compensated for by these odd number opticalamplifier repeaters AMP1, AMP3, . . . , is compensated for by the evennumber optical amplifier repeaters AMP2, AMP4, . . . , which areprovided with DCFs for each of the respective wavelength bands.Consequently for the overall optical communication system, dispersioncompensation can be performed for a wide band with even less DCFs, andhence an optical communication system with excellent transmissioncharacteristics at a high speed and over a wide band can be realized ata low cost.

With the optical communication system in FIG. 10, a plurality of opticalamplifier repeaters for compensating for wavelength dispersion whichcannot be completely compensated for with a single DCF are positionedone after the other. However, the present invention is not limited tothis, and at least one optical amplifier repeater of the plurality ofoptical amplifier repeaters can collectively compensate for thewavelength dispersion which has not been completely compensated for.However, since waveform distortion due to the influence of thewavelength dispersion occurs in a manner of distributed constant, thenas was the abovementioned embodiments, the more frequently thedispersion compensation is performed, the better the transmissioncharacteristics that can be obtained.

With the optical communication system in FIG. 10, each 1.58 μm bandoptical fiber amplifier can have an extra DCF, or a DCF providing anextra amount of dispersion compensation, to provide the additionalamount of dispersion compensation required by this wavelength band.Moreover, instead of providing additional dispersion compensation ineach optical fiber amplifier, the optical amplifiers can be arranged sothat each even (or odd) number 1.58 μm band optical fiber amplifier hasan extra DCF, or a DCF providing an extra amount of dispersioncompensation. Moreover, such optical amplifiers are not intended to belimited to 1.58 μm band. Instead, the principles of the presentinvention are applicable to other wavelength bands.

Although the optical communication system in FIG. 10 is described ashaving odd and even optical amplifiers having specific characteristics,the present invention is not intended to being limited to this specificarrangement of odd and even optical amplifiers. Instead, variouscombinations of the different types of optical amplifiers can bearranged along the transmission line. For example, a transmission linewith only a single optical amplifier of a different type than the otheroptical amplifiers along the transmission line might operatesufficiently for certain applications.

Furthermore, with the above embodiments of the present invention,examples are described where the WDM signal light is demultiplexed intotwo wavelength bands of 1.55 μm and 1.58 μm. However, the presentinvention is not limited to being multiplexed into two wavelength bands,or to any specific wavelength bands. Instead, for example, the WDMsignal light may be demultiplexed into wavelength bands other than thosementioned above or into three or more wavelength bands. The variousembodiments of the present invention can then be easily modified in viewof the required number of wavelength bands.

FIG. 13 is a diagram showing an example of optical fiber amplifier withan AGC circuit, according to an embodiment of the present invention.More specifically, FIG. 13 shows an example of the respective opticalfiber amplifiers and the AGC circuit shown, for example, in FIGS. 3, 4,6-9, and 11.

Referring now to FIG. 13, the light of respective wavelength bands inputfrom the left is branched by a first branching coupler 12, received by afirst photodiode (PD) 62, and the input light level is detected by anAGC control circuit 64. On the other hand, the light sent to an erbiumdoped fiber (EDF) 66 and amplified is branched by a second branchingcoupler 68, received by a second PD 70, and the output light level isdetected by AGC control circuit 64.

AGC control circuit 64 controls the light output level of a first laserdiode (LD) 72 and a second LD 74 serving as excitation light sources forexciting EDF 66, based on the light level received by first PD 62 andsecond PD 70, so that the gain of the optical fiber amplifier becomes apredetermined gain (normally a constant gain). The outputs from first LD72 and second LD 74 are respectively input to EDF 66 by a first WDMcoupler 76 and a second WDM coupler 78.

Optical isolators (ISO) 80 are respectively provided between firstbranching coupler 12 and first WDM coupler 76, and between second WDMcoupler 78 and second branching coupler 68. First LD 72 and second LD 74can be, for example, a 0.98 μm band laser and a 1.48 μband laser,respectively. Although the present invention is not intended to belimited to any specific wavelengths or wavelength bands, in variousembodiments of the present invention described herein, it is preferableto use a 0.98 μm band laser for first LD 72, and a 1.48 μm band laserfor second LD 74, to amplify signals in the wavelength bands describedin examples herein.

FIG. 13 represents only one example of an optical amplifier with an AGCfunction. The present invention is not intended to be limited to thisspecific example, and many different variations are possible.

Variable optical attenuators in which the light attenuation amount canbe varied by electrical control can be used as the variable opticalattenuators of FIGS. 3, 4, 6-9 and 11. For example, it is desirable touse an optical attenuator wherein the attenuation amount can be variedby controlling a Faraday rotation angle of a magneto-optic crystal.However, the variable optical attenuators are not intended to be limitedto such examples, and different types of variable optical attenuatorscan be used.

As described above, an optical amplifier can perform amplification bydemultiplexing wide band light into a plurality of wavelength bands,while incorporating a wavelength dispersion compensator inside theoptical amplifier which can compensate for wavelength dispersion for therespective wavelength bands.

In addition, by inputting lights of adjacent bands from different endportions of a dispersion compensating fiber, and making the propagationdirections of the lights of the respective wavelength bands inside thedispersion compensating fiber opposite, then the occurrence of thenonlinear optical effects inside the dispersion compensating fiber isreduced, enabling stabilization of the transmission characteristics.

Furthermore, in the case where the dispersion compensation for therespective wavelength bands cannot be completely compensated for by asingle wavelength dispersion compensator, by providing a secondwavelength dispersion compensator which separately compensates for thewavelength dispersion which could not be compensated for, for eachwavelength band, dispersion compensation for all of the wavelength bandscan be performed inside a single optical amplifier.

According to the above embodiments of the present invention, a DCF isused as a dispersion compensator. However, the present invention is notintended to be limited to the use of a DCF. Instead, other types ofdispersion compensators can be used in various embodiments of thepresent invention.

With an optical communication system according to embodiments of thepresent invention, then of the plurality of optical amplifier repeaters,for the optical amplifier repeaters having a first construction, acomparatively simple construction can be achieved by using a singlewavelength dispersion compensator. With the wavelength dispersion whichcould not be compensated for by these optical amplifier repeaters, byperforming separate compensation for each wavelength band, in theoptical amplifier repeaters of a second construction, then dispersioncompensation can be performed with respect to the respective wavelengthbands for all of the plurality of optical amplifier repeaters.Consequently, it is possible to realize a low cost for an opticalcommunication system incorporating a wavelength dispersion compensationfunction.

With a wavelength dispersion compensation method for wavelength divisionmultiplexed signal light of the present invention, by inputting signallights of adjacent wavelength bands from different ends of a dispersioncompensating fiber so that the propagation directions of the respectivesignal lights inside the dispersion compensating fiber are in oppositedirections, the probability of the occurrence of nonlinear opticaleffects in the dispersion compensating fiber can be reduced, so thatstabilized transmission characteristics can be obtained.

Optical amplifiers according to the above embodiments of the presentinvention relate, for example, to a case where amplification processingis performed for WDM signal light transmitted at, for example 10 Gb/s,with a transmission speed in excess of 2.5 Gb/s per unit wavelength, andwavelength dispersion compensation is performed inside the opticalamplifier. However, the embodiments of the present invention are notlimited to such specific examples. For example, the embodiments of thepresent invention are not limited to such transmission speeds.

According to the above embodiments of the present invention, an opticalamplifier includes respective optical fiber amplifiers for amplifyingdifferent wavelength bands. For example, in various embodiments of thepresent invention, the optical amplifier includes a 1.58 μm band opticalfiber amplifier 3 for amplifying signals in the 1.58 μm band, and a 1.55μm band optical fiber amplifier 2 for amplifying signals in the 1.55 μmband. However, the embodiments of the present invention are not intendedto be limited to having two optical fiber amplifiers for two wavelengthbands. Instead, the embodiments of the present invention can have aplurality of optical fiber amplifiers for amplifying a plurality ofwavelength bands. Thus, the present invention is not limited to anyspecific number of wavelength bands.

Further, according to various aspects of the present invention, all thecomponents of the optical amplifier can be enclosed in the same housing.For example, in FIG. 3, 1.58 μm band optical fiber amplifier 3, 1.55 μmband optical fiber amplifier 2, DCF 11, and WDM couplers 10 and 12 canbe enclosed in the same housing. WDM couplers 1 and 4 could also beenclosed in the housing, if desired. By enclosing the components in asingle housing, the overall apparatus can be packaged and sold as adiscrete component optical amplifier. Moreover, if the housing is madeof an appropriate material, the optical amplifier could be used inoptical submarine systems.

In addition, in various embodiments of the present invention, thepre-stage optical amplifier section of the optical amplifier preferablyoutputs a signal with sufficient power so as to be received by thepost-stage optical amplifier section without requiring furtheramplification between the stages. This way, further amplification is notrequires between the stages. In addition, preferably, the pre-stageoptical amplifier section and the post-stage optical amplifier sectionof each optical amplifier are connected together without SMF between thesections. For example, in FIG. 3, preferably, DCF 11 is between thepre-stage optical amplifier section and the post-stage optical amplifiersection, without SMF between the sections.

Although a few preferred embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

What is claimed is:
 1. An apparatus comprising: a demultiplexerdemultiplexing a wavelength division multiplexed (WDM) light into firstand second lights corresponding to different wavelengths in the WDMlight; a first amplification stage amplifying the first light, variablyattenuating the amplified first light, providing dispersion compensationto the attenuated, amplified first light, and amplifying the dispersioncompensated first light, to output a first stage light; a secondamplification stage amplifying the second light, variably attenuatingthe amplified second light, providing dispersion compensation to theattenuated, amplified second light, and amplifying the dispersioncompensated second light, to output a second stage light; and amultiplexer multiplexing the first and second stage lights together. 2.An apparatus as in claim 1, wherein each of the first and secondamplification stages comprises a dispersion compensating fiber forproviding dispersion compensation.
 3. An apparatus comprising: ademultiplexer demultiplexing a wavelength division multiplexed (WDM)light into first and second lights corresponding to differentwavelengths in the WDM light; a first amplification stage amplifying thefirst light, variably attenuating the amplified first light, providingdispersion compensation to the attenuated, amplified first light, andamplifying the dispersion compensated first light, to output a firststage light; a second amplification stage amplifying the second light,providing dispersion compensation to the amplified second light, andamplifying the dispersion compensated second light, to output a secondstage light; and a multiplexer multiplexing the first and second stagelights together, wherein the first amplification stage comprises a firstoptical amplifier amplifying the first light, a variable opticalattenuator variably attenuating the first light as amplified by thefirst optical amplifier, a dispersion compensator providing dispersioncompensation to the first light as amplified by the first opticalamplifier and as attenuated by the variable optical attenuator, and asecond optical amplifier amplifying the dispersion compensated firstlight.
 4. An apparatus comprising: a demultiplexer demultiplexing awavelength division multiplexed (WDM) light into first and second lightscorresponding to different wavelengths in the WDM light; a firstamplification stage amplifying the first light, variably attenuating theamplified first light, providing dispersion compensation to theattenuated, amplified first light, and amplifying the dispersioncompensated first light, to output a first stage light; a secondamplification stage amplifying the second light, variably attenuatingthe amplified second light, providing dispersion compensation to theattenuated, amplified second light, and amplifying the dispersioncompensated second light, to output a second stage light; and amultiplexer multiplexing the first and second stage lights together,wherein the second amplification stage comprises a first opticalamplifier amplifying the second light, a variable optical attenuatorvariably attenuating the second light as amplified by the first opticalamplifier, a dispersion compensator providing dispersion compensation tothe second light as amplified by the first optical amplifier and asattenuated by the variable optical attenuator, and a second opticalamplifier amplifying the dispersion compensated second light.
 5. Anapparatus comprising: a demultiplexer demultiplexing a wavelengthdivision multiplexed (WDM) light into first and second lightscorresponding to different wavelengths in the WDM light; a firstamplification stage amplifying the first light, variably attenuating theamplified first light, providing dispersion compensation to theattenuated, amplified first light, and amplifying the dispersioncompensated first light, to output a first stage light; a secondamplification stage amplifying the first light, variably attenuating theamplified second light, providing dispersion compensation to theattenuated, amplified second light, and amplifying the dispersioncompensated second light, to output a second stage light; and amultiplexer multiplexing the first and second stage lights together,wherein the first amplification stage comprises a first opticalamplifier amplifying the first light, a variable optical attenuatorvariably attenuating the first light as amplified by the first opticalamplifier, a dispersion compensator providing dispersion compensation tothe first light as amplified by the first optical amplifier and asattenuated by the variable optical attenuator, and a second opticalamplifier amplifying the dispersion compensated first light, and thesecond amplification stage comprises a first optical amplifieramplifying the second light, a variable optical attenuator variablyattenuating the second light as amplified by the first opticalamplifier, a dispersion compensator providing dispersion compensation tothe second light as amplified by the first optical amplifier and asattenuated by the variable optical attenuator, and a second opticalamplifier amplifying the dispersion compensated second light.
 6. Anapparatus comprising: a demultiplexer demultiplexing a wavelengthdivision multiplexed (WDM) light into first and second lightscorresponding to different wavelengths in the WDM light; a firstamplification stage amplifying the first light, providing dispersioncompensation to the amplified first light, and amplifying the dispersioncompensated first light, to output a first stage light; a secondamplification stage amplifying the second light, providing dispersioncompensation to the amplified second light, and amplifying thedispersion compensated second light, to output a second stage light; anda multiplexer multiplexing the first and second stage lights together,wherein the first amplification stage comprises a variable opticalattenuator attenuating the amplified first light before being providedwith dispersion compensation.
 7. A apparatus comprising: a first opticalamplifier amplifying a first light; a first variable optical attenuatorvariably attenuating the amplified first light; a first dispersioncompensator providing dispersion compensation to the attenuated,amplified first light; a second optical amplifier amplifying thedispersion compensated first light; a third optical amplifier amplifyinga second light having a different wavelength than the first light; asecond variable optical attenuator variably attenuating the amplifiedsecond light; a second dispersion compensator providing dispersioncompensation to the attenuated, amplified second light; a fourth opticalamplifier amplifying the dispersion compensated second light; and amultiplexer multiplexing the first light amplified by the second opticalamplifier and the second light amplified by the fourth optical amplifiertogether into a wavelength division multiplexed light.
 8. An apparatusas in claim 7, wherein the first and second dispersion compensators aredispersion compensating fibers.
 9. An apparatus as shown in claim 7,wherein attenuation of the first variable optical attenuator iscontrolled to maintain a power of the first light at an output of thesecond optical amplifier to be constant.
 10. An apparatus as in claim 7,wherein attenuation of the second variable optical attenuator iscontrolled to maintain a power of the second light at an output of thefourth optical amplifier to be constant.
 11. An apparatus as in claim 7,wherein: attenuation of the first variable optical attenuator iscontrolled to maintain a power of the first light at an output of thesecond optical amplifier to be constant; and attenuation of the secondvariable optical attenuator is controlled to maintain a power of thesecond light at an output of the fourth optical amplifier to beconstant.
 12. An apparatus as in claim 7, further comprising at leastone of: a gain controller controlling a gain of the first opticalamplifier to be constant, a gain controller controlling a gain of thesecond optical amplifier to be constant, a gain controller controlling again of the third optical amplifier to be constant, and a gaincontroller controlling a gain of the fourth optical amplifier to beconstant.
 13. An apparatus as in claim 11, further comprising at leastone of: a gain controller controlling a gain of the first opticalamplifier to be constant, a gain controller controlling a gain of thesecond optical amplifier to be constant, a gain controller controlling again of the third optical amplifier to be constant, and a gaincontroller controlling a gain of the fourth optical amplifier to beconstant.
 14. An apparatus comprising: a demultiplexer demultiplexing awavelength division multiplexed (WDM) light into first and second lightscorresponding to different wavelengths in the WDM light; a first opticalamplifier amplifying the first light; a first variable opticalattenuator variably attenuating the amplified first light; a firstdispersion compensator providing dispersion compensation to theattenuated, amplified first light; a second optical amplifier amplifyingthe dispersion compensated first light; a third optical amplifieramplifying the second light; a second variable optical attenuatorvariably attenuating the amplified second light, a second dispersioncompensator providing dispersion compensation to the attenuated,amplified second light; a fourth optical amplifier amplifying thedispersion compensated second light; and a multiplexer multiplexingtogether the first light after being amplified by the second opticalamplifier and the second light after being amplified by the fourthoptical amplifier.
 15. An apparatus comprising: a demultiplexerdemultiplexing a wavelength division multiplexed (WDM) light into firstand second lights corresponding to different wavelengths in the WDMlight; a first optical amplifier amplifying the first light, but not thesecond light; a first variable optical attenuator variably attenuatingthe amplified first light, but not the second light; a first dispersioncompensator providing dispersion compensation to the attenuated,amplified first light, but not providing dispersion compensation to thesecond light; a second optical amplifier amplifying the dispersioncompensated first light, but not the second light; a third opticalamplifier amplifying the second light, but not the first light; a secondvariable optical attenuator variably attenuating the amplified secondlight, but not the first light; a second dispersion compensatorproviding dispersion compensation to the attenuated, amplified secondlight, but not providing dispersion compensation to the first light; afourth optical amplifier amplifying the dispersion compensated secondlight, but not the first light; and a multiplexer multiplexing togetherthe first light after being amplified by the second optical amplifierand the second light after being amplified by the fourth opticalamplifier.
 16. An apparatus comprising: means for demultiplexing awavelength division multiplexed (WDM) light into first and second lightscorresponding to different wavelengths in the WDM light; means foroptical amplifying the first light separately from the second light;means for variably optically attenuating the amplified first lightseparately from the second light; means for providing dispersioncompensation to the attenuated, amplified first light separately fromthe second light; means for amplifying the dispersion compensated firstlight separately from the second light; means for amplifying the secondlight separately from the first light; means for variably opticallyattenuating the amplified second light separately from the first light;means for providing dispersion compensation to the attenuated, amplifiedsecond light separately from the first light; means for amplifying thedispersion compensated second light separately from the first light; andmeans for multiplexing together the amplified, dispersion compensatedfirst light and the amplified, dispersion compensated second light. 17.An apparatus comprising: a demultiplexer demultiplexing a wavelengthdivision multiplexed (WDM) light into first and second lightscorresponding to different wavelengths in the WDM light; a firstamplification unit comprising a first optical amplifier amplifying thefirst light, a variable optical attenuator variably attenuating theamplified first light, a dispersion compensator providing dispersioncompensation to the attenuated, amplified first light, and a secondoptical amplifier amplifying the dispersion compensated first light; asecond amplification unit, separate from the first amplification unit,comprising a first optical amplifier amplifying the second light, avariable optical attenuator variably attenuating the amplified secondlight, a dispersion compensator providing dispersion compensation to theattenuated, amplified second light, and a second optical amplifieramplifying the dispersion compensated second light; and a multiplexermultiplexing together the first light after being amplified by thesecond optical amplifier of the first amplification unit and the secondlight after being amplified by the second optical amplifier of thesecond amplification unit.